Well all seems to be sorted now. Seems the gasket between the throttle body and manifold was leaking, also the seal around the MAF sensor and housing wasn't good either. So after fixing the leaks and filling up with some fresh Shell V-Power, the car is running normal again - no cutting out, misfiring, dead throttle, etc. are all gone.
It seems a combination of boost and air leaks, plus loose dirty plugs all contributed to the issues. I'm also wondering if using Tesco 99 fuel contributed too - poor fuel, dunno?